Adding tool support for quantities

[erikbosch]

Take advantage of that VSS standard catalog now contains a quantity file. With this change an information message is given if there is no matching quantity-definition for a unit-definition. For now I opted to use "info" level, we may later raise it to "warning" or possibly even "error" for standard catalog PRs.

Output for standard catalog:

erik@debian4:~/vehicle_signal_specification$ make csv
./vss-tools/vspec2csv.py -I ./spec --uuid -u ./spec/units.yaml ./spec/VehicleSignalSpecification.vspec vss_rel_$(cat VERSION).csv
INFO     Output to csv format
INFO     Known extended attributes: 
INFO     Added 29 quantities from /home/erik/vehicle_signal_specification/spec/quantities.yaml
INFO     Added 60 units from ./spec/units.yaml
INFO     Loading vspec from ./spec/VehicleSignalSpecification.vspec...
INFO     Calling exporter...
INFO     Generating CSV output...
INFO     All done.

Example on message given if quantity is not defined:
(Quantity mazz used by unit lbs has not been defined)

erik@debian4:~/vehicle_signal_specification$ make csv
./vss-tools/vspec2csv.py -I ./spec --uuid -u ./spec/units.yaml ./spec/VehicleSignalSpecification.vspec vss_rel_$(cat VERSION).csv
INFO     Output to csv format
INFO     Known extended attributes: 
INFO     Added 29 quantities from /home/erik/vehicle_signal_specification/spec/quantities.yaml
INFO     Quantity mazz used by unit lbs has not been defined
INFO     Added 60 units from ./spec/units.yaml
INFO     Loading vspec from ./spec/VehicleSignalSpecification.vspec...
INFO     Calling exporter...
INFO     Generating CSV output...
INFO     All done.